1. Field of the Invention
This invention relates to optical glass suitable for molding the same by mold pressing and having optical constants of a refractive index (Nd) of from 1.50 to 1.60 and an Abbe number (.nu.d) ranging from 62 to 73,and further having a transformation temperature (Tg) of 550.degree. C. or lower.
2. Description of the Related Art
Recently, a mold pressing-technique in which a softened gob (glass-lump) is press molded to produce directly optical elements has actively been developed. Increasly, there has been a need for such optical glass having characteristics of softening capability at a low temperature, i.e. a low transformation temperature (Tg), excellent chemical durability, and good releasability from metal molds when pressing the glass. A variety of phosphate optical glass products have been known heretofore as having optical constants of a refractive index (Nd) of from 1.50 to 1.60 and an Abbe number ranging from 62 to 73. In general, phosphate system glass has poor chemical durability, such that a variety of proposals for improving this defect have been made. For instance, low refractive index and low dispersion optical glass products having excellent chemical durability have been known from Japanese Patent Laid-open No.118045/1985 and No. 171244/1985. However, these glass products tend to be easily cracked in press molding. Furthermore, Japanese Patent Laid-open No. 218941/1991 discloses phosphate optical glass for use in a laser device. However, this optical glass sticks easily to metal molds when the glass is subjected to press molding.
U.S. Pat. No. 2,423,128 discloses a phosphate glass which has the composition consisting of the following, by weight percent; 50-75 P.sub.2, O.sub.5, 2-10 SiO.sub.2, 8-16 Al.sub.2 O.sub.3, 1-10 B.sub.2 O.sub.3 and 2-10 CaO, wherein said glass is substantially colorless and easily melted, and has no tendency toward crystallization during reforming. However, because this glass contains no alkali metal oxide at all, the transformation temperature of the glass is relatively high and therefore not suitable for the purpose of the present invention.
U.S. Pat. No. 4,391,915 discloses a phosphate glass which has the composition consisting of the following, by weight percent 45-55 P.sub.2 O.sub.5, 3.5-9 Al.sub.2 O.sub.3, 5-20 K.sub.2 O, 0-3 Li.sub.2 O, 8-15 BaO, 4.5-9ZnO, 0-6 MgO, 0-18 PbO, 0-1 SiO.sub.2, 0-3 B.sub.2 O.sub.3 and 0-2 TiO.sub.2, wherein said glass is suitable for producing precision pressed optical components. This glass, however, is different in its essential ingredients from the glass of the present invention and in particular, has a lower P.sub.2 O.sub.5 content, is an important ingredient as a network former in the glass of the present invention.
Fluorophosphate optical glass containing fluorine ingredient has a low dispersion property and is advantageous also from the standpoint of softening capability at a low temperature. In the fluorophosphate optical glass, however, the fluorine ingredient vaporizes during press molding at a high temperature and produces fog on the surface of a metal mold such that this fluorophosphate optical glass is unsuitable for an optical glass which is subjected to mold pressing.
It is, therefore, an object of the present invention to provide optical glass with improved properties such that the glass barely cracks when it is subjected to mold pressing,and has good releasability from metal molds, maintaing optical constants of a refractive index (Nd) of from 1.50 to 1.60 and an Abbe number (.nu.d) ranging from 62 to 73 exhibits excellent chemical durability, and has softening capability at a low temperature.